The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In the field of an automatic transmission, more multiplicity of shift-stages is useful technology for enhancement of fuel consumption and drivability of a vehicle.
Achieving more shift stages contributes to improve performance and efficiency of a vehicle in a trend so-called downsizing of an engine, and more linearity in step ratios of the shift-stages enhances drivability of the vehicle by improving uniformity of acceleration before and after a shifting, etc.
In order to achieve more shift-stages for an automatic transmission, the number of parts is typically increased, which may negatively affect installability, production cost, weight and/or power flow efficiency.
Therefore, in order to enhance fuel consumption of an automatic transmission having more shift-stages, it is desired that better efficiency is derived with less number of parts.
In this aspect, an eight-speed automatic transmission has been introduced recently and a planetary gear train for an automatic transmission enabling more shift-stages has been investigated.
A recent eight-speed automatic transmission typically shows a gear ratio span, which is a significant factor for securing linearity of step ratios, in a level of 6.5 to 7.5.
In addition, we have discovered that an eight-speed automatic transmission may become inferior in respect of linearity of step ratios of shift-stages even if the gear ratio span may rise a level above 9.0.